In a film forming process performed in manufacturing various semiconductor devices, a temperature control is vital to secure a desired characteristic of a thin film and a high precision in film thickness. Any problem in the temperature control can directly lead to deterioration of the quality or reliability of final semiconductor devices.
A film forming apparatus for forming a film on a semiconductor wafer (hereinafter, also referred to as “wafer”) as a substrate to be processed by employing, e.g., a CVD method includes a susceptor serving as a substrate mounting table for mounting the wafer thereon, wherein the susceptor is formed of, e.g., a ceramic-based material such as AlN having a high thermal conductivity. While heating the wafer indirectly by way of heating the susceptor with heating units such as resistance heaters, various film forming reactions are made to be performed. Further, for the purpose of controlling the temperature of the wafer with a high precision during the film formation, the resistance heaters are grouped into two to correspond to, e.g., a central portion and a peripheral portion of the wafer, respectively, so that a heat transfer to the wafer can be conducted efficiently. Further, by installing a temperature detecting unit such as a thermocouple in the susceptor, the temperature distribution in the surface of the wafer can be improved and processing temperature uniformity among wafers can be realized.
However, if types of wafers (i.e., types of films formed on the wafers, types and concentrations of doped impurities therein, etc.) are different, thermal absorptances of the wafers can get different, so that it has been difficult to select an optimal temperature condition for every wafer when processing different types of wafers successively.
In practice, when heating different types of wafers successively, the temperature of the susceptor serving as a stage heater for heating the wafers behaves markedly differently depending on the presence or absence of films formed on the wafers, as shown in FIG. 1. Such behavior of the temperature of the susceptor is conjectured to be resulted from differences in thermal characteristics of the wafers, particularly, their thermal absorptances. Further, even in case the types of the wafers are identical, there may occur a difference in behaviors of the temperatures of, e.g., a central region and a peripheral region of the susceptor corresponding to the central portion and the peripheral portion of the wafers, respectively, which would result in deterioration in quality of the wafers after the film formation.
FIG. 3 is a graph showing measurement results of resistivities of a central portion (one position) and a peripheral portion (four positions) of each wafer after performing film formations on same types of wafers successively. As can been seen from FIG. 3, as the number of wafers processed increases, variations in resistivity become grater at the peripheral portions of the wafers while variations of resistivity at the central portions of the wafers are kept smaller. Variations in film qualities of wafers resulted from such in-surface temperature variations of the wafers can also be affected by external factors such as loading/unloading of the wafers, fluctuations of pressure in a processing chamber, deposits in the processing chamber, and so forth.
Accordingly, when selecting the temperature conditions, optimal heating conditions need to be selected by taking differences in positions on the surfaces of the wafers as well as differences in film types of the wafers into consideration.
As a technique for conducting a temperature control during a wafer processing, there is proposed a method for measuring infrared emissivity of a wafer and controlling a heating condition based on the measurement result (see, e.g., Japanese Patent Laid-open Publication No. 2003-45818: Reference 1, and Japanese Patent Laid-open Publication No. H6-158314: Reference 2).
In References 1 and 2, heating conditions are controlled on the basis of the types of wafers by measuring emissivities of the wafers. In these methods, though it is possible to improve inter-wafer reproducibility of processing, the technique to perform different controls depending on the areas of the wafers is not considered at all. Thus, the methods are insufficient to improve in-surface uniformity in a wafer (e.g., in-surface uniformity in quality and thickness of films formed on the wafer).
Further, since the susceptor typically has a temperature detecting unit such as the thermocouple, it is possible to perform a feedback control for a heater output while detecting a processing temperature. However, in case the temperature detecting unit cannot be installed at other locations than the central region of the susceptor due to a restriction from, e.g., a heater structure, it is impossible to correctly detect temperatures at the susceptor's other regions (for example, a peripheral region of the susceptor) where no temperature detecting unit is installed while the temperature of the central portion can be measured. As described above, since the temperature variation of the wafer tends to be great at the peripheral portion of the susceptor where no temperature detecting unit is provided, there is required a solution to this problem. With the conventional methods, however, it is difficult to correct heating conditions while improving the in-surface uniformity of the processing temperature for the wafer.